[rtve] Add support for audio only

[kebianizao]

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• Bug fix
• Improvement
• New extractor
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Description of your pull request and other information

This adds support to audio only urls to the rtve extractor. So far this extractor did not support urls like Audio only episode . With this extension such audio only url is supported.

The class supporting the above mentioned url includes a test case which runs successfully here.

Taken over from ytdl-org/youtube-dl#29023

[kebianizao]

Thanks for the review. I think I've addressed all comments, but let me know if you see anything else.

[pukkandan]

everything else looks good